Spectroscopic studies of systems in which iron porphyrins "activate" small molecules particularily dioxygen are proposed. These studies are designed to provide firm chemical models for the mechanism of action of a number of metalloenzymes including cytochrome P-450, monoxygenases, dioxygenases and peroxidases. Intermediates which have been previously proposed as important to the mode of action of these enzymes will be prepared at low temperature, their electronic structure will be characterized by 1H NMR, esr, infrared and electronic spectra, and their chemical reactivity will be monitored. The reactivity of peroxide bridged iron(III) dimers toward the addition of various bases and toward thermal decomposition will be examined. Methods of making peroxide bridged iron(III) dimers with improved thermal stability will be sought. The possibility of generating the ferryl ion (FeO2 ion) from peroxobridged iron(III) dimers will be examined. The ability of copper ions and complexes to coordinate to an iron bound dioxygen ligand will be examined. Chemical and electrochemical oxidation of porphyrin complexes of mu-peroxo iron(III) dimers and of ferryl ions will be utilized to prepare models for Compound I of horse radish peroxidase. Superoxide ion complexes of iron(II) porphyrins will be prepared via electrochemical reduction of iron porphyrin dioxygen adducts or by titration of superoxide ion with iron(II) porphyrins. The production of radicals via reduction of iron(III) porphyrins by amines, thiols and other organic molecules will be examined and the mechanisms of these reactions will be determined. The mechanism of autoxidations of organic molecules catalized by iron porphyrins will be studied.